Power amplifiers are widely used in communication systems, for example in radio base stations and cellular phones of a cellular radio network. In such cellular radio network, power amplifiers typically amplify signals of high frequencies for providing a radio transmission signal. A consideration in the design of power amplifiers is the efficiency thereof. High efficiency is generally desirable so as to reduce the amount of power that is dissipated as heat. Moreover, in many applications, such as in a satellite or a cellular phone, the amount of power that is available may be limited due to powering by a battery, included in e.g. the satellite. An increase in efficiency of the power amplifier would allow an increase of operational time between charging of the battery.
Outphasing type of power amplifier consists of two conventional amplifiers interacting with each other in order to keep Radio Frequency (RF) voltage as close maximum as possible and RF currents as low as possible. In more detail, it is the RF voltage and current at intrinsic transistor that is kept at maximum and minimum, respectively, in average over a signal amplitude distribution. In this manner, an efficiency of the power amplifier is as high as possible. The maximum voltage at the intrinsic transistor is obtained by taking the voltage over load as vector sum of the voltages of the constituent amplifiers. The load voltage is adjusted by adjusting the relative phase of the signals from two amplifiers.
In a traditional outphasing power amplifier, RF voltages are kept at maximum all the time, e.g. over time varying envelope of amplified signal.
In a Chireix type of power amplifier, compensation reactance is introduced to reduce the reactive parts of the currents, i.e. in average over signal amplitude distribution.
Even more efficient solution for outphasing is a power amplifier disclosed in U.S. Pat. No. 2,269,518A. This power amplifier was developed by Chireix and Fagot in the 1940s. With the power amplifier thus disclosed two amplifiers are driven in a linear fashion, with constant phases, for low amplitudes below a breakpoint. Outphasing occurs for amplitudes higher than the breakpoint. Accordingly, the breakpoint is set where the outphasing begins. The breakpoint may be set during design of the power amplifier.
WO2004/057755 discloses a combination of a peak amplifier of a Doherty amplifier and a pair of amplifiers forming a Chireix pair. This type of power amplifier has three optima in efficiency curve, and thus much higher average efficiency. Sometimes this kind of amplifier is referred to as a Chireix-Doherty power amplifier.
In U.S. Pat. No. 7,279,971B2 combination of multiple Chireix pairs is used. In order to reach the maximum efficiency up to four RF drive signals need to be used, e.g. two drive signals for each Chireix pair. Amplitude and phase as a function of output power are controlled.
In U.S. Pat. No. 6,825,719B1, generation of the 2 out-phasing drive signals, and the drive signals power control, e.g. for low power levels, is achieved with external control and phase shifters. The generation of the signals is complex and expensive due to required hardware and power consumption.
A problem with the above mentioned power amplifiers and drives therefor may be that they suffer from high complexity and cost. Moreover, another problem may be that the power amplifiers need external control and/or feedback.